[AlexN]

I use the one on my C11 about 1" from the corrector, The one for my Megrez 102 about 4.5~5" from the objective on the end of the extended dew shield... Both seem 100% spot on..

Heres a pic, Focused through the camera view finder (no angle adapter, so 0x magnification) with the mask 4 or 5 inches from the objective.